Numerous compositions that comprise at least two chemically distinct materials form stable liquid solutions when melted but separate spontaneously into two or more phases when solidified. Some compositions that are essentially a single chemical substance, although they are believed on theoretical grounds to have only one solid phase at equilibrium at any specified temperature and pressure, can form distinct phases other than or in addition to the equilibrium one when cooled quickly from melts. An important example of the latter type of material is alumina, which is known to be capable of existing in metastable "transition" phases such as gamma, delta, and theta when cooled rapidly from melts, even though its only equilibrium crystal form at normal temperature and pressure is believed to be alpha. Compositions of both these types will be denoted herein as "multi-phase solidifying compositions" or "MPSC's".
When melted MPSC's of the type with at least two distinct phases at equilibrium are cooled slowly, and when as is usual one of the stable solid phases has a higher equilibrium solidification temperature than the other(s), a very inhomogeneous solid structure usually results, because the slow cooling rate allows the formation of large domains of the first phase to solidify. These large domains are later embedded in a matrix of one or more additional phases after complete solidification.
When cooling of either type of MPSC is rapid enough, all the solid phases capable of coexistence at equilibrium will usually form quickly, so that the particles of each separate phase can be very small. In some cases, the equilibrium phase(s) may not form at all, leading to a metastable solid that is amorphous or that has such a fine microstructure as to appear for all practical purposes as an otherwise unknown "pseudo-phase", possibly comprising very small domains of all of the solid phases that would be formed from the same composition at equilibrium. Such metastable solids often have practically useful properties and are not readily or economically achievable by other techniques than rapid cooling. For example, U.S. Pat. No. 4,565,792 of Jan. 21, 1986 to Knapp teaches that rapidly solidified cofusions of zirconia and stabilizing oxides yield powders from which superior sintered ceramic bodies can be produced. The methods of cooling noted in this patent involve cooling in air or in thin layers on or between metal plates or spheres. European Patent Application No. 0 094 030 published Nov. 16, 1983 teaches the use of cold liquid jets to effect rapid cooling of various liquid compositions that yield ceramics upon cooling.
Additional methods of cooling known in the prior art are reviewed in Rapid Solidification of Ceramics, published by the Metals and Ceramics Information Center Battelle Columbus Laboratories, Columbus, Ohio in 1984. The part of this reference believed most relevant to this invention is that between pages 6-20 inclusive, and the most particularly relevant parts within this are FIGS. 2, 7, 8, 11, and 18 and the descriptive text associated with each of these figures. FIG. 2 shows a process in which solid powder is introduced into a plasmatron, in which the powder is presumably melted. The molten liquid droplets are then propelled onto the surface of a cooling metal disc by a flow of argon gas through the plasmatron. FIG. 7 shows a continuous stream of molten material flowing on to the surface of a rotating cylinder on which it is cooled and from which it may be detached as a continuous quenched film. FIG. 8 shows a similar stream of liquid flowing onto the inner surface of a rotating quench drum. FIG. 11 shows a process in which liquid from an "image floating zone" is propelled by blasts, presumably discontinuous, of high pressure gas, onto a water cooled substrate. FIG. 18 illustrates a process in which solutions of salts are blown in the form of a mist into a heated space in which the solvents evaporate and the salts decompose to yield fine powders. It is stated that this is not a rapid solidification process, but that it sometimes produces powders similar to those made by rapid solidification.
A particularly relevant comment from this Battelle reference is on page 19: "Based upon published literature it can be concluded that only laboratory scale processes have been developed for rapid solidification of ceramic materials."
A paper titled "Production and Processing of Rapidly Quenched Aluminum Powders" by R. E. Maringer, published at pages 67-74 of the record of the 25th National SAMPE Symposium and Exhibition held May 6-8, 1980, describes in its section 2.2.2 a method of making aluminum flakes by causing molten droplets of aluminum alloy to impinge on a cooled drum. This reference does not show any rapid cooling of ceramic materials, however, and it gives no detail about the method of atomization used.